An engineer, a mathematician, and a biologist are sitting outside of a building drinking coffee ... They see two people enter the building, and a minute later, the same two people come back out with a third person along side them. The biologist exclaims, ""they multiplied!"" The engineer comments, ""No, it must have just be a calculation error."" The mathematician finally said, ""Look, it doesn't matter how that third person got there, but what we do know is that if someone enters the building it will be empty again.""